Banks could lose money when they start to lose money in their investment ventures. As you know banks finance houses, cars, and basically every commodity that could generate income long as it's not perishable.
They don't have a single or just a few ventures so they get out of money if they lose money in some of their ventures, they don't lose money in the first place because they don't make investments or accept commodities and stuff that they know are not going to get them any profit in the first place, and even if they do it sometimes, they have a lot of ventures going on at one time and they earn more money than they might lose in some of those ventures.
So, this isn't more than an excuse if they say that they don't have any reserves left because they have lost money in their investment ventures because a bank can never get out of money because of their business structure which allows them to always have money in their reserves and they are supposed to have reserves for every single penny they have of their customers.